Bullets vary widely in size. Many pistols fire bullets that are only eight millimeters in diameter. In contrast, a 50-caliber rifle fires bullets that may be larger than an inch in diameter, and some guns, such as mounted anti-aircraft machine guns, may fire bullets that are several inches in diameter.

In a broad sense, a bullet differs from a missile primarily in the fact that a bullet does not have any method of guiding or propelling itself. Both the direction a bullet travels and the velocity it reaches are determined solely in and by the barrel of the firing mechanism. In contrast, a missile, broadly defined, generally carries a supply of propellant and may include a guidance system.